Today could be the big one for Gary Kasparov, the brash, impatient young challenger for the world chess championship.

Theoretically, it is possible for the championship match -- which is limited to 24 games -- to stretch out until Sunday. But today, as he sits down in Moscow's Tchaikovsky Concert Hall for Game 22 of the series, Kasparov is for the first time within striking distance of the title. A victory would give him the 12 1/2 points he needs; even a draw would move him within half a point of his goal.

It is still possible for defending champion Anatoly Karpov to keep his title. Among the wily Karpov's skills, none is more solidly established than his ability to hold on in a tight situation. He did that last week, snatching a draw in the face of what looked like a sure defeat.

But draws (a Karpov specialty) no longer are enough; he must win at least two of the three remaining games if he is to tie the match and remain champion. He has managed to win only two of the last 21 games, though he did it impressively: He took two in a row, games 4 and 5, after being stunned by Kasparov's brilliant victory in the opening game of the match.

Since then, Karpov often has defended brilliantly. But Kasparov has had the initiative most of the time. And his strength has seemed to grow as time passed and fatigue had a chance to work on his older opponent -- Kasparov is 22; Karpov is 34. After winning the first game, almost winning the second and then losing the fourth and fifth, Kasparov took a long time before chalking up his second victory, in Game 11. Since then, the pace of his victories has quickened, with a win in Game 16, another in Game 19 and a near-miss in Game 21. His victories have been spectacular -- considerably more interesting to the average chess fan than the slow, solid and methodical victories of Karpov. And even when the games were draws, Kasparov often made them exciting.

Can Karpov win two games in quick succession at the end of a long, exhausting series? Perhaps, but nobody except Karpov seems to be taking the possibility seriously. Karpov has reached the top by winning occasionally, but mostly by getting into a comfortable lead and then not losing it. In the decade he has reigned as champion, there have been many years when he played dozens of games, drew a large number, but lost only one or two. In this match, he already has lost four -- more than he has ever lost in such a short period since becoming champion, and probably more than he can afford to lose.

Still, the match is not quite over and might not end until Sunday. If Karpov wins both of the next two games, Game 24 on Saturday will be a sudden-death encounter, with Karpov playing for a draw and Kasparov going for a win. That is the best situation the champion can hope for at this point. If Karpov wins one and draws one in the next two games, he will be playing for the win on Saturday and Kasparov will be looking for a draw.

In any case, barring an outside intervention by the International Chess Federation (which cut off the first Karpov-Kasparov match without a decision), the last possible date for a final match game should be Saturday. Each of the players has exhausted the three postponements allowed him.

This is not only the first time Karpov has faced a challenger younger than himself; it is also the first time he has ever faced any challenger except Victor Korchnoi, a much older player who defected from the Soviet Union after losing his first challenge match, claiming that conditions had been rigged to favor the champion. In two subsequent encounters, in Switzerland and in the Philippines, the Karpov-Korchnoi rivalry plunged heavily into psychological warfare, with the psychodrama often getting more attention than the chess. There has been psychological play in the two Karpov-Kasparov matches, but mostly the two have played chess.

If Kasparov wins the title in Game 22, it will mark the end of a grueling ordeal: 70 games in a little over a year against the same opponent -- one with whom he had hardly played before their first match began in September 1984. This enormous effort contrasts dramatically with the way Karpov became champion 10 years ago. After winning the series of elimination rounds that made him the challenger, Karpov took the title without playing a single game against the reigning champion, Bobby Fischer, who forefeited his crown rather than agree to the match rules set up by the International Chess Federation.

Fischer never played another game in public after winning the championship from Boris Spassky in Iceland in 1972. After turning down offers that could have made him a millionaire, he withdrew into seclusion. Occasional rumors about his hermit-like existence in California still reach the media, and he is still a vivid presence in the minds of chess fans and players. But in terms of the power plays of international chess, he has ceased to exist.

Kasparov is the closest thing to Fischer the world has seen since the American-born champion abandoned his throne. He has a similarly brash, sometimes abrasive personality and a similarly sharp eye for winning combinations in situations where others see only routine possibilities. If he wins the championship this week, Kasparov will be the youngest champion in chess history, though only two years younger than Karpov was when he took the title. Fischer might have won the championship at an even younger age if he had accepted the routine processes of elimination in an orderly style. But that point (like so many points about Fischer) must remain a matter of speculation.

If the 70 match games between Karpov and Kasparov were considered as a single match, Karpov would be in a slightly more comfortable position than the challenger. Each of the players has won seven of those 70 games, and the champion keeps his title in the event of a tie. The first match was suspended by grandmaster Florencio Campomanes, president of the International Chess Federation, after Game 48, when Karpov was leading by a score of 5-3.

That match, following a pattern first demanded by Fischer, had no limit imposed on the number of games; the title simply would go to the first player who won six games. Karpov quickly came within striking distance of that goal. Then he simply stopped winning while Kasparov slowly rebuilt his strength and self-confidence in a long series of draws that was numbing to spectators but therapeutic for the challenger. Although Karpov was still ahead when the match was broken off, he had lost two games in a row. Chess fans widely interpreted the abrupt termination of the match as an unfair attempt to rescue the champion. Campomanes said it was done to avoid endangering the health of both players -- but at the time, Karpov was the only one who was looking ill.

Even more unfair, in the eyes of Kasparov and his supporters, was the change of rules for the second match, now nearing its end. The decision to limit this match to 24 games was seen as favoring Karpov's typical strategy (applied equally in matches and individual games), which is to win a small, early advantage, then relax, play defensively and wait for the opponent to disintegrate. With a limited number of games to play, time theoretically would be on the champion's side; if the first match had been played under the rules of the second match, Karpov would have won it without allowing his opponent to win a single game.

If Kasparov wins the title today, in Game 23 Thursday or in Game 24 during the weekend, he will be doing it under conditions that seem designed to favor his opponent's style of play. An ironic situation -- but in Moscow, irony seems to be almost a way of life.